Video of mouse gustatory ganglion neurons responding to tastants applied to the tongue. GCaMP fluorescence increases in intensity as calcium flows into the neurons, serving as a proxy measure of neural activity.

Research

Our lab is interested in investigating the sense of taste and the molecules, cells, and circuits involved in chemosensation from the tongue and gut to the brain. Taste receptor cells on the tongue are specialized to be activated by one of the five taste qualities, and signal that information to discrete populations of neurons in the gustatory ganglia through "labeled lines." This hard-wired, labeled line connectivity pattern is essential for our ability to correctly detect and discriminate tastes. The lab is interested in understanding how this gustatory circuit is organized at the cellular and molecular level.

Less well understood are chemosensory cells in the gut – which have many parallels to taste receptor cells – and may signal the presence of nutrients, toxins, and microbial metabolites to peripheral sensory neurons in the vagal ganglia. We aim to identify the cells and signaling mechanisms necessary for this gut-brain communication.

Techniques

The Macpherson Lab combines the power of mouse genetics with in vivo functional imaging of gustatory and vagal ganglia neurons. We use molecular cloning and BAC recombineering to engineer transgenic mouse lines for Cre-dependent expression of imaging and optogenetic toolkit genes (like GFP, GCaMP, and Channel Rhodopsin) within specific populations of cells. The lab also use CRISPR gene targeting to create knockout mouse models faster and easier than traditional methods. In addition to using circuit mapping techniques such as GFP Reconstitution Across Synaptic Partners (GRASP), the lab can manipulate these circuits with optogenetics, and assay the effect of their manipulation with behavioral assays.

Projects

Taste Connectivity

The connectivity between taste receptor cells (TRCs) in the oral cavity and axons from their partner neurons in the gustatory ganglia must be correctly established in order to respond appropriately (attraction vs. aversion) to a taste stimulus. Notably, TRCs turn over rapidly throughout the lifetime of the animal. How do bitter ganglion neurons recognize and reconnect to each new bitter taste receptor cell? Recently, we identified, for the first time, guidance cues which coordinate this process. We demonstrated that a Semaphorin (Sema3A) is selectively expressed by bitter TRCs to promote connectivity with bitter-responsive gustatory ganglion neurons, ensuring fidelity and specificity of the bitter ‘labeled line’.

We now are working toward uncovering the mechanisms by which gustatory ganglion neurons complete the ‘handshake’. Using an innovative approach combining mouse genetics, in vivo functional imaging, optogenetics, and behavior analysis, we propose to identify, characterize, and manipulate gustatory ganglion neurons.

Gut Chemosensation

Chemosensory signaling through the vagus nerve is an important component of the gut-brain axis, relaying information about the presence of nutrients and irritants within the lumen of the gut. Vagal sensory neurons transmit these signals from the gut to the brainstem, activating neural circuits to modulate the rate of digestion, or trigger emesis or diarrhea. Although the physiology of this circuit has been studied for decades, only recently have the molecular details of the receptors, cells, and signaling mechanisms begun to be defined. Basic information about the repertoire of vagal sensory neurons innervating the gut and the types of information they encode are lacking. We are interested in defining the chemosensory circuits for detecting nutrients and irritants in the gut through imaging, activity dependent labeling, circuit mapping, and molecular characterization of functional subsets of vagal sensory neurons.